How to clean “no clean” flux residues from printed circuit boards and do it right

Cleaning no clean flux residues remaining on a printed circuit board assembly (PCBA) is one of the most difficult cleaning processes.

By definition no clean flux residue are harder to clean than conventional rosin or modified resin based residues.  This is because the residue is not meant to be cleanable.

However, there are times when the residues do need to be cleaned.


So what do you do when you want to clean the no clean residues from the circuit?

Whether a flux residue can be cleaned effectively depends on the cleaning materials saponification factor and its compatibility with the no clean residues.

Saponification is the ability of the no clean residues to be softened to the point of being able to be dissolved by the alkali content (the saponifier) of the cleaning chemistry.

The higher the saponification factor of the cleaning fluid the easier it is to clean the residues. Also, if the saponifier is not compatible then it won’t work as well.

So the key here is to ensure that the saponifier is compatible and completely dissolves the residues.

What happens if the residues are only partially dissolved by the saponifier?

A no-clean residue that is only partly cleaned away could be far worse for a printed circuit board assembly than a no-clean residue left untouched from a corrosion point of view.

One of the reasons is because lead free flux activators are more active than those in earlier leaded flux formulations.

When un-cleaned the residues are locked up in the carrier resin matrix. They are stable (benign) at normal operational temperatures and therefore will not leach out dangerous residues and cause corrosion problems.

However, if the protective matrix around the residue is partially removed by an inadequate cleaning regime, then the activators could be exposed.

This may lead to a corrosion process starting on the circuit board and this process could be accelerated in the presence of heat, power on the boards in service or high relative humidity.

So how do you clean “no-clean” residues without problems?

It is important when considering cleaning “no-clean” residues on a circuit board that you check:

  • The ability of the residue to be cleaned is determined?
  • The cleaning chemistry is matched to the relative degree of difficulty and the available process.
  • The success of the whole process is validated by careful testing

Following these three guidelines can help you be successful. Not considering these three points could easily lead you to having real problems in the long term.

Need to find out more?

For further information on cleaning circuit boards for conformal coating then review our cleaning section or contact us directly.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating ebook.


What are the key questions to ask when cleaning for conformal coating?

Of all the questions I am asked related to cleaning the most frequent is

“Do I need to clean my circuit board before conformal coating?”.

If only it was simple to answer!

Generally, cleaning of a circuit board before conformal coating application is desirable. For Parylene this requirement may be even more important due to the bonding issues of the material.

However, it may not be essential.

Ultimately, the cleanliness of the circuit can be crucial for both the short-term production process and the long-term reliability of the circuit.

But, its difficult to know for sure if you need to clean until you complete some trials.

For the process it is difficult to know until you attempt to apply the conformal coating in production.

Then you will know immediately if you need cleaning since if you have problems you will see conformal coating defects like de-wetting, de-lamination and poor adhesion.

For the long-term reliability unless you want to wait until your product fails then some sort of reliability testing should be tried.

After all the purpose of the application of the conformal coating is to improve reliability.

It has less value unless the board is clean enough in the first place.

Other key questions in cleaning

There are many questions relating to conformal coating and cleanliness that are regularly asked.

These include:

  • What is being cleaned when circuit boards are washed?
  • How should I clean my electronic circuit board?
  • How do I validate my cleaning process?

If further information on these topics and the key question you can go to our free eBook by clicking Nexus conformal coating and cleaning.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page.

Dr Lee Hitchens, Author of Nexus website and ebook
Dr Lee Hitchens, Author of Nexus

Dr Lee Hitchens is the author of the Nexus conformal coating website and eBook.

Send me an email at and let me know what you think?


Nexus Series of Webinars: Conformal Coating Rework, Repair and In Process Application

Nexus are to hold a series of webinars on conformal coating topics in the Autumn of 2012.The series of four online conformal coating webinars will cover the following topics and each last 90 min. The last of the online training webinars will be:

Rework, Repair & In Process Application
Monday 4th February Rework of coating boards is a skilled job to obtain the optimum product reliability. Different materials or thicker coating may require different techniques to maintain the same reliability provided by the original coating system. It’s not always possible to use the same coating system in a rework situation so what are the alternatives?

Topics will include:

Material identification
Conformal Coating removal techniques
Cleaning & preparation
What coatings are compatible?
Re-coating assemblies after rework
Inspection of coatings after rework

Please click online conformal coating webinars for further information.